The present invention relates to apparatus and processes for continuous distillative separation of a mixture comprising one or more alkanolamine(s).
For the distillative, for example continuous, fractionation of multisubstance mixtures, various process variants are in common use. In the simplest case, the mixture to be separated (feed mixture) is fractionated into two fractions, a low-boiling top fraction and a high-boiling bottom fraction.
In the case of separation of feed mixtures into more than two fractions, a plurality of distillation columns have to be used in this process variant. In order to limit the apparatus complexity, columns with liquid or vaporous side draws are used if possible in the separation of multisubstance mixtures.
However, the possible use of distillation columns with side draws is greatly limited by the fact that the products withdrawn at the side draw points are rarely, if ever, completely pure. In the case of side withdrawals in the rectifying section of the column, which are typically effected in liquid form, the side product still comprises proportions of low-boiling components which should be removed via the top. The situation is similar for side withdrawals in the stripping section of the column, which are usually effected in vaporous form, in which the side product still has high boiler contents.
The use of conventional side draw columns is therefore limited to cases in which contaminated side products are permissible.
One means of remedy is that of dividing wall columns in which the side products can also be obtained in high purity (see, for example, FIG. 1). This column type is described, for example, in:    U.S. Pat. No. 2,471,134, U.S. Pat. No. 4,230,533, EP 122 367 A, EP 126 288 A, EP 133 510 A,    Chem. Eng. Technol. 10, (1987), pages 92-98,    Chem.-Ing.-Tech. 61, (1989), 1, pages 16-25,    Gas Separation and Purification 4 (1990), pages 109-114,    Process Engineering 2 (1993), pages 33-34,    Trans IChemE 72 (1994), Part A, pages 639-644, and    Chemical Engineering 7 (1997), 72-76.
In this design, a dividing wall is mounted in the middle region above and below the feed point and the side withdrawal, and seals the feed section 2, 4 from the withdrawal section 3, 5, and prevents cross-mixing of liquid and vapor streams in this column section. In the case of separation of multisubstance mixtures, this reduces the total number of distillation columns required. As in the case of conventional side draw columns, intermediate evaporators and intermediate condensers can also be used in dividing wall columns. Intermediate condensers are preferably mounted at the upper end of the dividing wall or in the common column region 1 above the dividing wall. Intermediate evaporators are preferably provided at the lower end of the dividing wall or in the common column region 6 below the dividing wall.
With the same energy consumption, a dividing wall column can also be replaced by the arrangement of thermally coupled distillation columns. A description of thermally coupled distillation columns, which can be designed in various apparatus configurations, can likewise be found in the abovementioned references in the technical literature. It is also possible to equip the individual component columns completely with evaporators and condensers. This corresponds to a dividing wall column with an intermediate evaporator and an intermediate condenser. A particular advantage of this specific configuration is that the individual columns can also be operated at different pressures. This enables excessively high temperature spreads to be prevented and the operating temperatures to be matched better to given heating and cooling media. The options for energy integration measures are improved.
In a specific configuration, in the case of dividing wall columns and thermally coupled distillation columns, it is also possible to withdraw two pure side fractions instead of one. The withdrawal section 3, 5 is extended by an intermediate column region 7 (FIG. 1a). It is also possible to provide, in column regions 1, 3, 7, 5 and 6 or between column regions 1 and 3 and 5 and 6, further side withdrawals which, however, cannot provide entirely pure fractions.
A further design of dividing wall columns usable in accordance with the invention envisages designing the dividing wall so as to be continuous either up to the upper or lower end of the distillation column (FIG. 1b). This design corresponds to the arrangement of a main column with an attached side column. In this embodiment, the expected advantages over conventional column arrangements are not energy advantages but capital cost advantages.
Dividing wall columns and thermally coupled distillation columns offer advantages over the arrangement of conventional distillation columns both with regard to the energy demand and to the capital costs.
For the regulation of dividing wall columns and thermally coupled columns, various regulation strategies are described. Descriptions can be found in:    U.S. Pat. No. 4,230,533, DE 35 22 234 C2, EP 780 147 A,    Process Engineering 2 (1993), 33-34, and    Ind. Eng. Chem. Res. 34 (1995), 2094-2103.